the opmixer-0.75.ebuild has a slight problem with dependencies... it depends on =gtkmm-1.2* but the ebuild says >=gtkmm-1.2.2 which means the emerge will fail on a fresh system, because it will try to emerge gtkmm-2.2 patch attached
Created attachment 7422 [details, diff] the patch that should fix the problem (untested by me) untested, but should work...
ok, fixed in the tree. Thanks for your report.